A ramp function generator is used to provide an electrical signal whose value increases linearly with time. Among other applications, such a signal can be used as a `demand` for a servo control loop such as that incorporated in a material testing machine. In a material testing application, a ramp function generator is used to demand, say, a load of a test specimen that increases at a steady rate. The important features of this demand signal are that:
A. It should be possible to stop, start, and reverse it when required. PA1 B. That, in the stop condition, the signal will not drift. PA1 C. That the ramp, i.e. the rate of variation of the signal with time, shall be variable over a very wide range. PA1 D. That the signal shall increase or decrease smoothly.
It is possible by analogue signal generation utilizing an integrating amplifier, to provide a smooth signal, i.e. one which increases in truly linear fashion with time and without steps, but the other requirements are not met.
A digital technique is therefore more frequently used, a typical example being shown in FIG. 1 of the accompanying drawings.
This includes a binary counter 100, having its input connected to a variable frequency oscillator 110 and its output connected to a digital-to-analogue (D/A) converter 120. The output signal of the converter approximates to the desired linearly increasing ramp signal AB (FIG. 2) but in fact consists of a series of discrete steps as shown greatly exaggerated in FIG. 2, the steps corresponding to the units counted in the counter. The number of steps therefore increases with increase in the number of bits in the counter and so approaches more closely to the ideal as the number of bits is increased. Apart from lack of smoothness of the output signal arising from the relatively low resolution, all the other requirements are met. The slope of the ramp may be varied by altering the frequency of the oscillator. The signal may be reversed by causing the counter to count negatively and may be stopped precisely. In the stop condition it will not drift. This is important in tests for creep.
The resolution presently available from a signal generator of this kind does not exceed 4000-8000 bits per ramp. While this is adequate for many purposes it does not suffice for the tensile and fatique testing of very stiff specimens.
With a view to improving the resolution we investigated the possibility of filling up the steps in the ramp by applying an additional voltage derived from an integrating amplifier. This, however, was not feasible because we found that, while the steps in the ramp occur at regularly timed intervals, they are not precisely equal in height, due to tolerances in resistors which are switched into and out of operation in the D/A converter and in the switches which effect this switching.